Many applications—laboratory, industrial, and audio applications being some examples—share the need to measure the difference between two relatively weak voltages on which there is a common voltage superimposed. Instrumentation amplifiers fill this need by rejecting the superimposed voltage, or common mode voltage, on two input voltages and generating an output voltage that is directly proportional to the difference between the two input voltages. Thus, the output voltage, VOUT, of an instrumentation amplifier is characterized by the operation of a common mode gain, ACM, and a differential gain, ADM, on the two input voltages, V1 and V2, according to the equation:VOUT=ADM(V1−V2)+ACM(V1+V2)/2
An ideal instrumentation amplifier has a constant differential gain and a common mode gain of zero. In practice, the common mode gain may not be zero but instead may be much less than the differential gain. An important measurement for instrumentation amplifiers is the common mode rejection ratio (CMRR), which is a ratio of the common mode gain to the differential gain expressed in decibels. Typical instrumentation amplifiers may have CMRR in the range of 30 to 60 dB, with higher CMRR being desirable.
Typical instrumentation amplifiers are implemented using operational amplifiers and four or more resistors. The magnitude of resistance presented by particular resistors in the instrumentation amplifier dictates the gain of the amplifier. However, controlling the gain of these amplifiers through resistor selection is typically contingent upon matching paired resistors. Fabrication may be difficult because of the constraint of precisely matching resistor pairs. For example, discrepancies between resistors may be alleviated through the expensive process of resistor trimming.
Resistors in instrumentation amplifiers may pose additional complications. For instance, resistors, such as polysilicon resistors, may have voltage coefficients that create significant distortion or non-linearity in the output voltage, even when resistor pairs match perfectly. Such voltage coefficients will allow any common mode voltage to effectively modulate the output voltage by changing the differential gain.